Niobium compounds are, amongst other applications, suitable to be used as dopants for certain catalysts or ceramic materials. Ceramic materials include electroceramics used in applications such as multi-layer ceramic capacitors, piezoceramics and ferrites. In this application, the catalyst carrier or active species will be mixed with the dopant and calcined in a subsequent process step. The calcination usually takes place under conditions where the niobium compound is converted to pure niobium pentoxide. Such applications encompass, but are not limited to, ethane hydrogenolysis, dehydrogenation of hydrocarbons, solid acid catalysis in polycondensation, or ammoxidation. Such catalysts in which niobium compounds are used are in particular catalysts to convert butane to acrylonitrile in the presence of ammonia and oxygen conversion of butane to acrolein in the presence of oxygen (oxidation), oxidative dehydrogenation of alkanes, like conversion of propane to propene. Such applications are, for example, disclosed in Chemical Reviews 1999, (99), 3603-3624, which is enclosed by reference for all useful purposes. Mixing with the niobium compound often takes place in an aqueous solution, suspension or emulsion.
For such applications it is important that the niobium compound exhibits excellent water solubility, yields pure niobium pentoxide with no impurities from complexing agents or counterions when calcined under appropriate conditions and has a high content of niobium in weight percent, so with addition of small amounts of the niobium compound an aqueous solution having a suitable high niobium concentration can be prepared.
Limar et al. in Russian Journal of Inorganic Chemistry, Vol. 9 No. 10, October 1964, pages 1288-1291, disclose the niobium oxalate complexes Na[Nb(O)(C2O4)2(H2O)]*4H2O as well as (NH4)3[Nb(O)(C2O4)3]*x H2O. These complexes, however, do not fully satisfy the conditions as set out above. With sodium as countercation, sodium impurities will remain in the niobium pentoxide after calcination. The ammonium complex is 3-fold negatively charged resulting in lower niobium content. Coordination with three oxalate ions has the same effect of lowering the niobium content in percent by weight. Additionally, the water solubility of merely 20 g Nb/1 and 40 g Nb/1, respectively, needs to be significantly improved.